Greek police and army officers stand by Greek, Russian and EU flags as they wait for the arrival of Russian President Vladimir Putin in Athens, Greece airport, May 27, 2016. REUTERS/Michalis Karagiannis/File Photo

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Greek police and army officers stand by Greek, Russian and EU flags as they wait for the arrival of Russian President Vladimir Putin in Athens, Greece airport, May 27, 2016. REUTERS/Michalis Karagiannis/File Photo

Athens: Greece will expel two Russian diplomats suspected of meddling in the politically sensitive issue of Macedonia, a Greek diplomatic source said on Wednesday, as NATO prepares to invite the former Yugoslav republic to join the alliance.

Russia said it would respond in kind to the Greek move, in a rare diplomatic tussle between two Orthodox Christian nations that have traditionally enjoyed warm relations.

Macedonia is expecting an invitation at the NATO summit in Brussels this week to join following its landmark deal with Greece whereby it will change its name to the Republic of North Macedonia.

Moscow strongly opposes NATO expansion.

A member of Russia's upper house of parliament, Andrei Klimov, told RIA news agency that Moscow would expel two Greek diplomats in response.

He did not mention Macedonia or give further detail about the matter.

The Greek diplomatic source told Reuters Athens would expel two diplomats and bar two other Russians from entering the country due to concerns that they were involved in rallies in Greece against the deal with Macedonia and that they had attempted to offer money to Greek state officials.

Athens raised its concerns with Moscow and asked the diplomats on July 6 to leave Greece, the source said, adding that they were given a "reasonable period" of a few days to pack their bags.

"These expulsions are justified because they concern illegal activities at the expense of our national security," the source said.

"However, the warnings of expulsions from the Russian side are unjustified because our (diplomats) act within the limits of their duties."

Greece and Macedonia have both seen many protest rallies against the name accord, particularly in the run-up to the signing of the accord by their prime ministers on June 17.

Athens and Moscow have traditionally had good relations. Greece did not join most other NATO allies in March in withdrawing diplomats from Moscow over the poisoning of a former Russian spy in England.

Britain blamed Russia for that incident. Moscow denied involvement.

"(This is) an amazing and highly significant development given close ties between Athens and Moscow," said James Ker-Lindsay, senior visiting fellow at the London School of Economics.

"It is a clear warning not to abuse friendship. Extraordinary!" he tweeted.

Moscow regards NATO enlargement as a direct threat to its own security. Many countries that were once in Moscow's orbit have joined NATO and the European Union since the fall of communism in 1989.

Greece had long blocked Macedonia's bid to join NATO and to start European Union membership talks because it says the country's name implies a territorial claim over a northern Greek province also called Macedonia.

Macedonia will only join NATO after its people have endorsed the deal with Greece in a referendum.